1. Field of the Invention
The present invention relates to a circuit for lighting a discharge lamp and, in particular, refers to an electronic ballast circuit for fluorescent lamps.
2. Description of the Related Art
Discharge lamps (for example, fluorescent lamps) provide light in numerous commercial, industrial, and consumer applications. The discharge lamps are illuminated when driven by an alternating current (AC) signal, such as signals from a power line which oscillate at a relatively low frequency (for example, 60 Hertz). The discharge lamps typically need a ballast circuit (for example, a magnetic ballast circuit) to interface with the power line. The ballast circuit for low frequency operation is generally bulky and operates the discharge lamps inefficiently.
Electronic ballast circuits have been introduced to increase power efficiency of the discharge lamps by converting the power line signal to a relatively higher frequency AC signal and driving the discharge lamps with the relatively higher frequency AC signal. The higher frequency AC signal requires less current to flow through the discharge lamps to achieve the same light output, and lower current flows can lengthen the life of the discharge lamps. Generally, electronic ballast circuits are much more expensive than magnetic ballast circuits.
Discharge lamps with filaments at opposite ends generally become inoperable when one or both filaments are worn out (or burned out). The burnt out discharge lamps are typically replaced with new discharge lamps. The burnt out discharge lamps need to be handled carefully because they may contain harmful elements, such as mercury. Improper handling during disposal of the discharge lamps can cause the mercury to inadvertently leak and contaminate the environment.
The present invention solves these and other problems by providing a compact, cost-effective, efficient, and reliable circuit which is compatible with existing lighting systems for discharge lamps. In one embodiment, an energy efficient ballast (or an electronic ballast) drives a discharge lamp, such as, for example, a T-8 or T-12 fluorescent lamp. The energy efficient ballast includes an inverter (or an oscillator or a converter) which accepts a substantially direct current (DC) input voltage and provides a substantially AC output voltage to drive the discharge lamp at a relatively high frequency. In one embodiment, the DC input voltage is provided by a full-wave rectifier circuit coupled to an AC power line. The amplitude of the DC input voltage or the AC power line can be varied to provide brightness control (or dimming) of the discharge lamp.
In one embodiment, the inverter includes semiconductor switches in a full-bridge (or an H-bridge) configuration. For example, a first semiconductor switch is coupled between a positive terminal of the DC input voltage and a first node. A second semiconductor switch is coupled between the first node and a negative terminal of the DC input voltage. A third semiconductor switch is coupled between the positive terminal of the DC input voltage and a second node. Finally, a fourth semiconductor switch is coupled between the second node and the negative terminal of the DC input voltage.
In one embodiment, the inverter includes semiconductor switches in a half-wave bridge (or push-pull) configuration. For example, a first semiconductor switch is coupled between a positive terminal of the DC input voltage and a first node. A second semiconductor switch is coupled between the first node and a negative terminal of the DC input voltage. The lamp load is provided between the first node and a neutral (e.g., a ground or virtual-ground) node.
The inverter also includes a feedback control circuit which senses the current through the discharge lamp to control the semiconductor switches. For example, a sensing element is coupled in series with the discharge lamp. In one embodiment, the feedback control circuit is a transformer, and the sensing element is a primary winding of the transformer. Secondary windings of the transformer are coupled to control inputs (or control terminals) of the semiconductor switches.
In one embodiment, the semiconductor switches are realized with bipolar transistors. For example, base terminals of the bipolar transistors are coupled to the respective secondary windings of the transformers. In one embodiment, respective resistors are coupled in series with the base terminals and emitter terminals to limit currents through the semiconductor switches to safe levels.
In one embodiment, the primary winding of the transformer is coupled between the first node and a first cathode (or an electrode or a filament) of the discharge lamp. A timing capacitor (or an initiating capacitor) is coupled between the first cathode and a second cathode of the discharge lamp. An inductor (or a choke coil) is coupled between the second cathode of the discharge lamp and the second node.
The semiconductor switches alternately conduct to provide the AC output voltage to the discharge lamp at a frequency determined by the timing capacitor and the inductor. For example, the first semiconductor switch and the fourth semiconductor switch operate as a first pair to provide a voltage of a first polarity to the discharge lamp. The second semiconductor switch and the third semiconductor switch operate as a second pair to provide a voltage of a second polarity to the discharge lamp.
In one embodiment, a start-up circuit is coupled to the inverter for reliable operations. The start-up circuit automatically provides a pulse (or a trigger signal) to the feedback control circuit of the inverter to initialize the sequence of operation for the semiconductor switches when necessary. For example, the trigger signal is provided to one of the secondary windings of the transformer or to the control terminal of one of the semiconductor switches.
In one embodiment, the start-up circuit includes a capacitor which charges at a relatively slow rate in comparison to the operating frequency of the inverter. The charging capacitor raises a voltage of an avalanche device which outputs the trigger signal when the voltage reaches a predetermined level. Once the inverter is operating, the start-up circuit is relatively inactive.
In one embodiment, a multi-lamp ballast operates multiple discharge lamps. The multi-lamp ballast includes a multi-lamp inverter, similar to the inverter described above, with a plurality of semiconductor switches in a full-bridge or half-bridge configuration and a feedback control circuit for operating the semiconductor switches. However, the multi-lamp inverter includes multiple timing capacitors and inductors. The timing capacitors are coupled across cathodes of each of the respective discharge lamps. The inductors are coupled in series with each of the respective discharge lamps. The inductor-capacitor-discharge lamp combinations are coupled in parallel for operation.
In one embodiment, a bypass circuit (or a back-up circuit or a redundant circuit) is coupled across leads (or pins or terminals) of a cathode of the discharge lamp to extend the life the discharge lamp, thereby reducing its disposal rate. The bypass circuit advantageously extends the life of the discharge lamp without retrofit. The bypass circuit is substantially inactive when the cathode is operational. When the cathode wears out or becomes inoperable, the bypass circuit automatically activates to provide a conductive path for continuing operation of the discharge lamp. In one embodiment, the bypass circuit includes a pair of diodes placed in parallel opposition.
In one embodiment, a thermistor serves to limit the current supplied by the electronic ballast oscillator when there is no discharge lamp.
These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.